1. Field of the Invention
The present invention relates to a light modulating material.
2. Description of the Related Art
In light modulating materials using liquid crystal, switching between a scattered (light-shading) state and a transparent state is performed by the following methods.
For example, a method in which microcapsules containing liquid crystal are dispersed in a polymer has been suggested (see e.g. “Development of Chromic materials”, edited by Kunihiro Ichimura, CMC Publishing, pp. 226-236, 2000). According to this method, when voltage is not applied, liquid crystal molecules align randomly and light scatters due to diffused reflection resulting from a difference in the refractive indices of the liquid crystal molecules and the polymer. When voltage is applied, the light modulating material becomes transparent because the liquid crystal molecules align in the same direction and the refractive indices of the liquid crystal molecules in a major axis direction and the polymer become substantially the same.
In the above method, it is necessary to dissolve dichroic dyes in the liquid crystal in order to color a liquid crystal device. However, this method is problematic in that the walls of the capsules may be stained with the dichroic dyes, or the voltage response may become nonexistent because the dichroic dyes have aligned along the polymer film too easily, resulting in a reduction of light transmittance of the light modulating material when in a transparent state.
Another method using a mixture of an uncured UV-curable resin, a polymerization initiator, liquid crystal, and a dichroic dye has also been suggested. In this method, the resin is cured by irradiation with UV light, and the polymer and the liquid crystal are phase-separated from each other such that an interface is formed between the polymer and liquid crystal. Accordingly, this method functions according to the same principle as the above-mentioned liquid crystal light modulating device (see, for example, WO 2002/093241 and WO 2004/005246).
However, this method is problematic in that the dye decomposes due to irradiation with UV light, or due to the presence of the polymerization initiator, resulting in deterioration in coloring properties.
In addition, in the above method, the light modulating material is in a colorless scattered state or a colored scattered state when voltage is not applied thereto. Therefore, it is impossible for the light modulating material to be in a transparent state when voltage cannot be applied thereto due to wiring problems, a discharged battery, or the like.
Contrastingly, a polymer dispersed liquid crystal (PDLC) is known in which liquid crystal and a polymer are phase-separated due to crosslinking of the polymer which resides between transparent electrodes, and a liquid crystal mixture into which a gelation agent is injected is also known. It is known that the polymer dispersed liquid crystal and the liquid crystal mixture are transparent when voltage is not applied thereto and scatter light when voltage is applied thereto (see, for example, “Liquid Crystals”, Vol. 29, pp. 295-300, 2002, and “Journal of Materials Chemistry”, Vol. 13, pp. 2870-2874, 2003).
However, these methods are not widely used because the light modulating materials employing the methods are white when they scatter light, and have poor light-shading properties.
In addition, in guest-host liquid crystal systems using dichroic dyes (see, for example, “Liquid Crystal Handbook” edited by the liquid crystal handbook editorial committee, Maruzen p. 498, 2000), when a concentration of a dye which resides between transparent electrodes is controlled so that the light transmittance in a transparent state is 60%, the light transmittance when it is changed to a colored state due to the application of voltage, is about 30%. Thus, this system has a poor shading property between these states.